Rotors are a commonly used in applications where it is desirable to compress or move a fluid and where it is desired to remove energy from the fluid. In one example, a compressor or supercharger utilizes a pair of rotors to increase airflow into the intake of an internal combustion engine. In another example, a volumetric fluid expander includes a pair of rotors that expand a working fluid to generate useful work at an output shaft. In such applications, it is known to provide machined or cast rotors having a unitary construction with a solid cross-sectional area. Improvements are desired.